In forming conventional piezoelectric/pyroelectric films, a solution casting method or a melt-stretching method is commonly used. According to both of the methods, a formed film undergoes a stretching process, a thermal process, and a process applying a high voltage to orient directions of electric dipoles of molecular chains in the film to one direction (polarization process). Thereafter the electrode is coated on both surfaces of the film using a sputtering method, a silk printing method or the like, to thereby obtain a piezoelectric/pyroelectric film.
As such a polarization method which applies the high voltage to the film after the solution casting and melt-stretching in forming the piezoelectric/pyroelectric films, FERROELECTRIC CAST FILM AND ITS MANUFACTURING METHOD (Japanese Unexamined Patent Application Publication No. 2010-64284) or NON-POROUS POLYVINYLIDENE FLUORIDE (PVDF) FILMS IN THE BETA PHASE AND PROCESSING METHOD THEREOF (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-501826) is disclosed.
However, both of the above publications have a large number of operation steps because after the film is formed, the high voltage is applied thereto and then the film is polarized. Also, since each of these employs the casting method, there is a limit in atomization of the solution. According to the Japanese Unexamined Patent Application Publication No. 2009-501826, the film is formed using the casting method and the press-stretching method, and hence there are three multiple processes of drying, pressing and stretching after cast coating. Also, it cannot deal with a three-dimensional and large-sized material.
Next, PRODUCTION OF ORGANIC PYROELECTRIC AND PIEZOELECTRIC BODY AND DEVICE THEREFOR (Japanese Unexamined Patent Application Publication No. Hei 07-11424) or METHOD FOR FORMING ORGANIC PYROELECTRIC AND PIEZOELECTRIC BODY (Japanese Unexamined Patent Application Publication No. Hei 05-311399) is disclosed as a conventional polarization method.
However, according to the above two publications, a material to be polarized (corresponding to a material to be coated according to the present invention) is polarized in batch units using a vacuum chamber, which makes it disadvantageous for continuous mass production. Moreover, in such a polarization device, a negative electrode for supplying a high voltage and forming electric field between the material to be polarized is fixed or movable only in a uniaxial horizontal direction. Therefore, a polarization film can only be formed on a planar-shaped material to be polarized, not on a three-dimensional material.
In these two publications, there is also a limit in atomization of the solution.
There is also a known method to perform spraying and polarizing simultaneously, by applying a high voltage directly to a dielectric solution inside a solution cylinder, and dropping the solution to a material to be coated, where an electric field is formed between the solution cylinder and the material to be coated, to thereby achieve polarization before the solution reaches the material to be coated.
This method is disclosed in METHOD AND APPARATUS FOR PRODUCING MOLECULAR CRYSTALLINE THIN FILM (Japanese Examined Patent Application Publication No. Hei 07-55300), PRODUCTION METHOD OF DIELECTRIC PARTICLE AGGREGATE (Japanese Unexamined Patent Application Publication No. 2010-228993) or NANOPARTICLE SYNTHESIS WITH ELECTROSPRAY USING RECTANGULAR AC HIGH VOLTAGE (EAROZORU KENKYU. 23(2), 94-100 (2008)).
According to such known methods, however, a dielectric solution, in which a dielectric substance is dissolved and dispersed in a solvent, has a low electrical resistance value (approximately 0 to 200 kΩ, close to water with impurities mixed therein). Therefore, when a high voltage is applied to the solution, an electric current leaks to a solution tank grounded through a solution supply route, whereby a voltage and electric field toward the material to be coated become lower. Also, when performing electrospray coating by using an industrial paint including an organic solvent, an appropriate electrical resistance value of the solution is supposed to be 5,000 to 10,000 kΩ. According to the above known methods, an appropriate polarization process is not possible because of the low voltage and electric field toward the material to be coated.
Moreover, according to the method and device of NANOPARTICLE SYNTHESIS WITH ELECTROSPRAY USING RECTANGULAR AC HIGH VOLTAGE, electricity leakage is easily caused when applying a high voltage, which makes it difficult to realize consistent production. In order to apply a high voltage stably to a material to be polarized in a mass-producing device, it is necessary to dispose a solution supply route and a high voltage apply route separately, causing the device to increase in size.
In the above three publications, there is also a limit in atomization of the solution.
In a method of forming a piezoelectric/pyroelectric film by spraying or dropping a dielectric solution, electric dipoles in the solution are oriented by electric field. Thereafter, it is necessary to allow the dielectric solution to be in a semi-cured state or a solidified state by quickly evaporating the solvent during the period from when the solution is sprayed until it adheres to the material to be polarized or after it adheres to the material to be polarized, in order to avoid the relaxation of the orientation.
In order to realize the above, it is a known method to generate an air stream of an inert gas to a direction different from a flow of a sprayed solution, so that the evaporation of the solvent is facilitated. An example of this method is MONOLITHIC PYROELECTRIC INFRARED IMAGE SENSOR USING PVDF THIN FILM (R&D Review of Toyota CRDL, Vol. 33, No 1, 1998.3).
According to this example, however, there is also a limit in atomization of the solution and a large unit is necessary to maintain an atmosphere formed by the gas by enclosing the entire device.